Use MatDateSelectionModel to model the selected value in MatDatepickerInput

[jaguima]

We had to change one test in order to pass. Please take a look if the change is ok or if other changes would be required.

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[mmalerba]

Please make the commit description reflective of what is actually happening in the commit, e.g. "Use MatDateSelectionModel to model the selected value in MatDatepickerInput"

[mmalerba]

This looks good to me aside from the one comment above, and the fact that CI is red. You can run the following command to fix the api_golden_checks: bazel run //tools/public_api_guard:material/core.d.ts_api.accept, I think the other failure might just be a flake

[mmalerba]

The renaming CI failure is definitely a flake or existing issue, this PR does not affect the mat-menu.

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